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Not all combinations, however, are equally convenient or efficient.

The most common isotope, Pu-239, is produced when the most common isotope of uranium, U-238, absorbs a neutron and then quickly decays to plutonium.

The Pu-240 has a high spontaneous rate of fission, and the amount of Pu-240 in weapons-grade plutonium generally does not exceed 6 percent, with the remaining 93 percent Pu-239.

Higher concentrations of Pu-240 can result in pre-detonation of the weapon, significantly reducing yield and reliability.

Pu-239 in the order of 90-95 %, is known as weapon-grade plutonium.

Plutonium containing lower concentrations, in the range of 50-60 % is known as reactor-grade plutonium.

Such brief irradiation is quite inefficient for power production, so in power reactors the fuel is left in the reactor much longer, resulting in a mix that includes more of the higher isotopes of plutonium ("reactor grade" plutonium).

Normally for electrical power production the uranium fuel remains in the reactor for three to four years, which produces a plutonium of 60 percent or less Pu-239, 25 percent or more Pu-240, 10 percent or more Pu-241, and a few percent Pu-242.

While this yield is referred to as the "fizzle yield," a one-kiloton bomb would still have a radius of destruction roughly one-third that of the Hiroshima weapon, making it a potentially fearsome explosive.

Regardless of how high the concentration of troublesome isotopes is, the yield would not be less.

Such physical characteristics make reactor-grade plutonium difficult to manipulate and control and therefore explain its unsuitability as a bomb-making ingredient.

The isotope plutonium-238 would typically consitute only 0.036 percent of weapons-grade plutonium.

It is this plutonium isotope that is most useful in making nuclear weapons, and it is produced in varying quantities in virtually all operating nuclear reactors.